Internal combustion engines are well known and widely used for power generation, vehicle propulsion, and virtually innumerable other purposes. In a typical internal combustion engine design a mixture of fuel and air is delivered into one or more cylinders formed in an engine housing, to ignite and drive linear motion of the piston by way of hot and expanding combustion gases. The linear motion of the piston is transformed into rotational motion of a crankshaft in a well-known manner. Combustion of the fuel and air produces engine exhaust which is discharged from the engine system commonly after one or more forms of exhaust aftertreatment.
Depending upon the type of internal combustion engine and other factors such as jurisdictional regulations it can be desirable to reduce, trap, eliminate or chemically transform various constituents of engine exhaust in the aftertreatment system. In the case of compression ignition diesel engines and the like, exhaust constituents in the nature of oxides of nitrogen or “NOx,” particulate matter, and unburned hydrocarbons are desirably minimized during engine operation and commonly treated in the aftertreatment system, so long as engine performance and efficiency are not unduly impacted.
A great many different strategies for controlling combustion of fuel and air within an engine as well as treating exhaust that is produced have been proposed over the years. Strategies to cool the combustion process to reduce NOx production, such as by introducing recirculated exhaust gas, are used in some systems. Other strategies rely upon geometry of engine components, such as pistons, to affect NOx production or particulate matter production. It has been observed that many strategies require a shifting of a balance between NOx production and particulate matter production, such that production of these exhaust constituents is at least somewhat inversely related. Tilting the balance of NOx to particulate matter too much in one direction or the other can, however, create new challenges in the attempt to solve others. Manipulation of the combustion process within an engine can also undesirably affect efficiency or performance.
As noted above, various techniques can be used downstream from the engine to treat whatever exhaust constituents are desired to be reduced or eliminated. Many modern engines employ a so-called selective catalytic reduction module to reduce NOx (“SCR”), and a diesel particulate filter (“DPF”) to reduce NOx and particulate matter, respectively. A diesel oxidation catalyst (“DOC”) oxidizes organic constituents and carbon monoxide. The SCR module typically requires an on-board supply of reductant, and the DPF typically needs some mechanism or specialized engine operating technique to periodically regenerate. It will be readily appreciated that the interplay of the various technologies and available techniques for engine exhaust treatment and control of the combustion process is complex and sometimes unpredictable. It will also be recalled that degradation of performance or reductions in efficiency can accompany specialized engine operating techniques and specialized exhaust treatment equipment. The general complexity of such challenges is often reflected in the high level of sophistication employed in engine calibration. Presented with a group of many cross-coupled variables and non-linear relationships among engine operating parameters, engineers will often calibrate an engine to optimize to only one or a small number of criteria. Narrow and focused calibration or general operating strategies can have their advantages, but are not without shortcomings. By way of example, U.S. Pat. No. 5,067,460 to Van Duyne is directed to a variable air/fuel ratio engine, and proposes controlling a spark-ignition engine where fuel efficiency is maximized over an entire range of operating conditions.